The state of the art discloses exhaust-gas treatment devices in which, upstream from an exhaust-gas converter, the exhaust-gas path is divided by an inner tube into a central flow path and a peripheral flow path (e.g. German patent application DE 103 50 516 A). One of these parallel flow paths, especially the peripheral flow path, contains an exhaust-gas treatment means. Suitable actuators serve to convey the exhaust-gas stream through the annular exhaust-gas treatment means via the peripheral flow path or else to bypass the latter via the central flow path. Such an arrangement is known, for example, in conjunction with a downstream three-way catalyst upstream from which there is an adsorber for hydrocarbons (HC adsorber) in one of the parallel flow paths. As long as the main catalyst has not yet reached its operating temperature after the engine has been started, the exhaust-gas stream is conveyed through the peripheral HC adsorber that temporarily stores the hydrocarbons present in the exhaust gas. As soon as the downstream catalyst has reached its operating temperature, the exhaust-gas stream is conveyed via the central flow path so as to bypass the HC adsorber and to heat it up. Once the HC adsorber has reached its desorption temperature, the exhaust gas is once again conveyed partially or completely through the HC adsorber in order to discharge the released hydrocarbons and to transport them into the main catalyst, where they are catalytically converted.
A disadvantage of the arrangement described above is that the exhaust-gas stream impinges upon the downstream main catalyst non-homogeneously and also differently, depending on whether the exhaust gas has been conveyed via the central flow path or via the peripheral flow path. So far, no measures are known that are capable of remedying this problem.
German patent application DE 37 33 402 A relates to the problem of the non-homogeneous flow that reaches a catalyst arranged in an exhaust-gas pipe whose diameter is smaller than that of the catalytic converter. The catalyst substrate is situated in a catalyst housing that is connected to the exhaust-gas pipe via an inlet tube (diffuser) that widens conically in the flow direction of the exhaust gas. In order to homogenize the flow that reaches the catalyst element, several flow baffles that divide the exhaust gas path into several parallel exhaust-gas paths are provided upstream from the catalyst. For instance, the flow baffles can comprise several concentrically arranged plates in the form of truncated cones having an increasing opening angle.
Another approach is proposed by German patent specification DE 198 39 754 B. According to this approach, a baffle element configured as a vane is arranged in the exhaust-gas feed pipe connected to the inlet funnel of the catalyst housing and this baffle element imparts a swirl to the exhaust-gas stream in order to homogenize the flow that reaches the catalyst.
German patent application DE 198 38 703 A deals with the problem of an exhaust-gas flow which displays a strong swirl generated by the turbine of an exhaust-gas turbocharger and which causes a non-homogenous flow to reach the front face of a catalyst located downstream. As a remedy, a baffle in the form of a compressed “8” is arranged in the exhaust-gas pipe.